* {
    margin: 0;
    padding: 0;
}

.container {
    position: absolute;    /*绝对定位，原始的元素框从文档流中删除*/
    top: 50%;
    left: 50%;
    margin-top: -100px;
    margin-left: -200px;
    width: 400px;
    height: 200px;
    background-color: #ccc;
}

.left-top-area {
    width: 50px;
    height: 50px;        
    border-bottom-right-radius: 50px;
    background-color: #fc0;
}

.right-bottom-area {
    position: relative;    /*相对定位，元素框相对与原来的位置偏移，原先的位置保留*/
    top: 100px;
    left: 350px;
    width: 50px;
    height: 50px;
    border-top-left-radius: 50px;
    background-color: #fc0;
}